There are many instances when there is a need to maintain information in a confidential manner safe from inadvertent or intentional viewing and yet have it available for almost immediate access when required.
There are a number of so-called disappearing inks and the like which have been formulated to keep writings a secret, but these do not necessarily protect the document and will not serve to protect writings or other subject matter generated by ordinary writing and printing means. U.S. Pat. Nos. 3,451,143 and 3,632,364 describe a variation on instructional sheets formed by spirit duplication to conceal information which is later made visible by a color change.
U.S. Pat. No. 4,038,761 describes use of the color-change capability of liquid crystals to show a correct answer in selected portions of the sheet whenever the temperature of a liquid crystal segment reaches its transition temperature and changes color or becomes transparent to show information keyed to the selected portions of the sheet.
It is also known to use the liquid crystal sheets for the purpose of providing temperature scales by exposing temperature markings or the like under a sheet of liquid crystal having temperature transition states changing in correspondence with the portion of the underlying scale exposed as the temperature changes. U.S. Pat. No. 4,891,250 teaches the use of a layer of nematic crystal material to provide color changes with temperature to monitor the operating temperature of electrical components.